In a cellular mobile radiotelephone system, a collection of cells or radio base stations each having a specified coverage area is controlled to provide duplex radio communications with a large number of mobile stations throughout a geographical area. The coverage areas or cells of the individual base stations overlap to provide continuous coverage. Responsibility for mobile stations in overlapping areas is assigned based on various criteria to define in effect a patchwork of non-overlapping contiguous cells, typically illustrated as hexagons. The base stations are linked to a mobile switching center which is in turn linked to the public telephone system to allow the radiotelephone system to function as an extension of the conventional telephone system.
Hot a call connection to or from a mobile station to remain uninterrupted as the mobile station travels from cell to cell, responsibility for handling the call connection must be "handed-off" from one cell to another. To accomplish such handoff, a procedure must be provided for locating the mobile station, or tracking its position in relation to cell boundaries.
Various methods are known for locating mobile stations in a mobile radiotelephone system. Two such methods of particular importance involve signal strength monitoring and "path-loss" monitoring, respectively. Signal strength and path loss are related but distinct quantities. Signal strength is a measure of the power of a received radio frequency signal. Path loss is a measure of the decrease in power of a radio frequency signal during transit of the signal from transmitter to receiver. Path loss may be calculated from signal strength if transmission power is known.
Most present-day mobile radiotelephone systems use signal strength to determine the location of mobile stations. When the signal strength of a mobile station falls below a specified threshold, for example, the mobile station is assumed to nave passed beyond the boundaries of the currently-responsible cell. The mobile station is determined to have entered the neighboring cell that registers the highest signal strength for the mobile station.
One mobile radiotelephone system, the GSM system, is known to use path loss for locating the position of mobile stations and enjoys several important advantages over position locating techniques using signal strength. By providing that a mobile station is always associated with the cell that results in the lowest path loss for that connection, communications may be achieved using the least possible power. Also, base station transmission powers may be adjusted (for example increasing the power of a base station experiencing problems being properly received by mobile stations) without affecting cell boundaries as would happen using signal strength. Also, using path loss instead of signal strength, uniform cell boundaries are applicable to mobile stations transmitting at different power levels. Using signal strength, a call connection with a mobile station transmitting at high power will be maintained at a greater distance than a call connection with a mobile station transmitting at a lower power such that the effective cell size is different for mobile stations of different power classes.
The use of path loss alone to locate mobile stations may in some instances, however, be disadvantageous. If, for example, an established call connection enjoys little or no signal-strength margin (excess signal strength above the bare minimum required to sustain communications), and if a different criterion than signal strength is used to determine the need for handoff, then the call connection may be interrupted when its signal strength margin has been exhausted without a handoff having been arranged. The foregoing class of calls may be referred to as being signal-strength limited, or "carrier-to-noise" (C/N) limited. When a call connection is not C/N limited, the path loss criterion may be advantageously employed. Such calls are more susceptible to interference from other calls than to low signal strength and may be referred to as "carrier-to-interference" (C/I) limited. The path loss criterion minimizes interference by assuring that the greatest possible amount of radiated energy reaches its intended destination.
What is needed, then, is a mobile radiotelephone system that takes advantage of the benefits of using path loss to make locating and handoff decisions, including the benefits of power conservation, low interference, and fixed cell boundaries, and that at the same time avoids the disadvantages associated with the use of path loss, including possible call connection interruption and loss of the call.